The descriptions used by SQL Doc are stored as extended properties, which in turn are stored in the database itself. As a result they will be backed up & restored within your database.

So the next question is, How to get all the extended Propeties out of the database?
I would use SQL Compare to compare my database against a blank database. And export the data as XML...
I would then use XSLT or a program to scan the XML and strip out the sp_addextendedproperty.
The XML looks like

The above example was taken from AdventureWorks.
Once I had this working well I would setup up some command line utlitilies to automate this task as part of my general build mechanisms...
Hope this helps
David Connell